I can imagine they could learn a lot about surviving in space; that alone would be worth it, to advance science. I mean they can achieve a lot of it already on earth (and have, with simulated moonbases and the like), but there's nothing like doing it the hard way.

I believe we need a moon base first before attempting to send people to Mars. Both are super dangerous, but at least you can reach and return from the moon within days if need be. I'm afraid the first mars missions will be one-way trips - and the astronauts will be aware of this as well.